Various methods for extracorporeal blood treatment or cleaning are used in chronic kidney failure in order to remove substances usually eliminated with urine and for fluid withdrawal. In hemodialysis, the patient's blood is cleaned outside the body in a dialyzer. The dialyser comprises a blood chamber and a dialyzing fluid chamber, which are separated by a semipermeable membrane. During the treatment, the patient's blood flows through the blood chamber, whilst dialyzing fluid flows through the dialyzing fluid chamber in order to free the patient's blood from substances usually eliminated with urine.
Whereas the transport of the low-molecular substances through the membrane of the dialyzer is essentially determined by the concentration differences (diffusion) between the dialyzing fluid and the blood in the case of hemodialysis (HD), substances dissolved in the plasma water, in particular higher-molecular substances, are effectively removed by a high fluid flow (convection) through the membrane of the dialyzer in the case of hemofiltration (HF). In hemofiltration, the dialyzer functions as a filter. Hemodiafiltration (HDF) is a combination of the two processes.
Various dialyzers or filters are known for performing extracorporeal blood treatments. The known dialyzers or filters include the so-called high-flux and low-flux dialyzers or filters. High-flux dialyzers are characterized by a higher ultrafiltration coefficient than low-flux dialyzers.
For an extracorporeal blood treatment, the doctor in charge must preselect a number of treatment parameters which on the one hand relate to the patient and on the other hand to the blood treatment apparatus used to perform the blood treatment. The treatment parameters to be preselected by the doctor are therefore referred to below as patient-specific or machine-specific treatment parameters.
In the following, patient-specific treatment parameters are understood to mean the parameters which are characteristic of the therapeutic aim and/or the patient to be treated. The doctor in charge first selects the respective therapeutic method for the patient, a distinction having to be made for example between hemodialysis (HD), hemofiltration (HF) and hemodiafiltration (HDF). Furthermore, the doctor establishes treatment time t, taking account of the individual patient's circulatory stability. Furthermore, the doctor establishes dialysis dose kt/V as the target value for the treatment. A value for dialysis dose kt/V of 1.4 generally applies as the target value for a suitable treatment. The literature also discusses higher target values for women or particularly light patients. A further patient-specific treatment parameter is the flow of blood in the extracorporeal circuit. The blood flow can be varied by the doctor within certain limits during the treatment, but ultimately the adjustable blood flow is dependent on the nature and the properties of the patient's vascular access.
In the following, the machine-specific treatment parameters are understood to mean the parameters which the doctor himself can freely select for the performance of the extracorporeal treatment, taking account of the patient-specific treatment parameters. The machine-specific treatment parameters include in particular the dialyzing fluid flow rate, which can be set freely within certain limits with the known extracorporeal blood treatment apparatuses.
Since the doctor in charge has various types of dialyzer or filter available to him for the performance of the extracorporeal blood treatment, which include for example the aforementioned high-flux and low-flux dialyzers, the doctor must select a specific dialyzer or filter before the treatment. The selection of a particular type of dialyzer, however, in turn affects the machine-specific treatment parameters which the doctor must set in order to achieve the therapeutic aim. With an unchanged blood flow, the same clearance can be achieved with a dialyzer characterized by a greater effective surface with a smaller dialyzing fluid flow rate as with a dialyzer characterized by a smaller effective surface.
The doctor in charge will select the patient- and machine-specific treatment parameters in such a way that the blood treatment can be carried out in the optimum manner for the given patient. When the doctor in charge has different types of dialyzer or filter available to him, however, there may be different parameters with which the blood treatment can be carried out in the optimum manner.